Retone repo into a self-guided walkthrough

[aj-enns]

Why

The repo was written as a facilitator-led Lunch & Learn: speaker scripts, minute-by-minute timings, "talking points," and notes addressed to "attendees." That made it awkward for an individual developer to just clone it and learn the GitHub Copilot app on their own. This reframes everything for a self-paced reader following along solo.

What changed

All changes are documentation; no application code was touched.

• Reframed the voice across the README and all six module docs. Removed Time: ~X min, "Talking points," "Point to make," "A good narration," "Do it live," "read it aloud," "recap slide," "project this on screen," and the "Land the transition" speaker cues. Replaced them with reader-facing sections like "Try it," "What's happening," "Why it matters," and "The mental model."
• Renamed docs/facilitator-guide.md to docs/walkthrough.md and rewrote it as a "start here" self-paced path instead of a run sheet, updating all references.
• Prerequisites no longer say "send this to attendees" and the facilitator pre-flight is now a personal quick checklist.
• Exercises drop the "demo"/"during the talk"/"attendee exercise" framing.
• Renamed the demo-task issue template to practice-task and switched the seed-issue label from demo to practice, updating the gh issue create commands and the tip to match.
• CONTRIBUTING.md and sample-app/README.md drop "Lunch & Learn," "workshop," and "repeatable demos" language.

Notes for reviewers

• The file renames are tracked as renames (docs/facilitator-guide.md -> docs/walkthrough.md, demo-task.md -> practice-task.md), so the diffs are content edits rather than full delete/add.
• I verified no leftover presentation framing remains and no internal links point to the old filenames.
• The intentional sample-app gaps and their exercises are unchanged, so main stays green.